Project Summary Inflammatory bowel disease (IBD), which includes Crohn?s disease and ulcerative colitis, is a devastating autoimmune disease that afflicts over 1 million Americans. IBD develops when genetically predisposed individuals mount an abnormal immune response to commensal and symbiotic intestinal bacteria. The crucial players in the initiation of IBD pathology are antigen-presenting cells (APCs) and commensal-specific CD4+ T cells. A key unanswered question in this field is how do genetic polymorphisms affect the control of APCs and commensal-specific CD4+ T cells? The lack of suitable models to address this important question has hampered progress in this field. We have developed a novel antigen-specific IBD model against a commensal bacterium, which provides a unique opportunity to dissect each step in the complex cellular and molecular interactions between immune cells and the intestinal microbiota. Based on loss of responses to two inhibitory cytokines that have been implicated in IBD pathogenesis, IL-10 and TGF-?, we have generated a CD4- dnTgfb2; IL10rb-/- mouse model of IBD called dnKO that develops severe disease upon colonization with the commensal bacterium Bacteroides thetaiotaomicron (B. theta). Recent work from our lab demonstrated that B. theta interacts with host immune cells through its outer membrane vesicles (OMVs), which are 40-60 nm spherical buds of the outer membrane from Gram negative bacteria. OMVs were found within host macrophages in the lamina propria of B. theta colonized dnKO mice, suggesting that B. theta OMVs drive disease by delivering antigens to the host APCs, not intact bacteria. In vivo, we have generated a 6B1 TCR transgenic mouse strain that recognizes a B. theta OMV antigen. In our model we know the specificity of the Teff and Treg and can control the genetics of the T cells and the host APCs. We propose to use our novel 6B1 B. theta-specific CD4+ T cell model to determine the mechanisms by which genetic mutations in APCs and commensal-specific T cells drive the initiation of IBD. We hypothesize that genetic mutations in the T cells and the APCs are both responsible for the initiation of colitis. To test this hypothesis, we aim to: 1) ascertain how colonic APCs process and present B. theta OMVs and how IL-10 controls the ability of OMVs to stimulate 6B1 T cells and initiate colitis and 2) determine the contribution of TGF? and IL-10 in preventing colitis induced by 6B1 T cells. Our findings should deepen our understanding of immune regulation in the intestine and may reveal therapeutic targets to prevent the induction of colitis.